...pithy Aspect phrases is an art that I'm still learning. One tip so far: when a player Aspect describes a skill or mindset, try rephrasing to describe how they got that way. (Allow players to rewrite aspect phrasing in the first couple of sessions if something better comes up.) Example from play: "Muscled and charges in" on our white council apprentice got way better once we changed it to "I was the college quarterback."

The main room is lined with model vintage cars, used DVDs, obscure action figures, a life-like cat statue with rabbit fur, and a table of beat-up comics. Its main income is obviously trading cards. There are no dice beyond the standard cheap plastic d4/6/8/10/12/20, the D&D section seems to have been abandoned to its own devices a couple years ago, the miniature selection is minimal, odd, and exclusively plastic.

And the gaming room is lined with (I was told) enough supplies to feed a family of ten for a year.

A friend of mine is running a campaign and wants to make a political twist. As he knows I use to play that gamestyle (although in a contemporary setting), he has asked me to get involved on the setting design. The campaign will be based on a city state in Middle Earth norwest.
I have made up an ...

I think it's simultaneously a little too open-ended and a little too specific. The answers you've gotten are ignoring the exact request you made because it's so specific, and are struggling with being relevant to your needs because of the open-ended nature of your framework.

Your specific request is for "simple, but well thought sources (books, articles,...) for advice on medieval political campaigns."

@Flamma So, given that medieval politics has very little in common with the Middle Earth setting (the point of religion is a major one, as any medieval or Renaissance social or political discussion is going to be steeped in religion), I think people are very confused about what might answer your specific question.

I suppose the crux of the matter is: Are you looking for Middle Earth material, or medieval material? Which is the real foundation of your campaign?

@Flamma Say exactly what you mean: That you're looking for information to help you run a political campaign in a Middle-Earth city-state, and you're specifically hoping for discussions of using real-life examples, perhaps from Medieval Europe.

I am interested in guidelines and sources of advice for designing political fantasy campaigns. They don't have to be Middle-earth specific, but ideas about converting them to Middle-earth would also be helpful.

A friend of mine is running a Middle-earth campaign and wants to make a political twist. As he knows I use to play that gamestyle (although in a contemporary setting), he has asked me to get involved on the setting design. The campaign will be based on a city state in Middle-earth norwest.
I am ...

@Phil Lessee... Tiny bit of D&D 2E, quite a bit of 3E and 3.75 and some 4E. Mutants and Masterminds, GURPs (eww), touched on Deadlands/Savage Worlds, Exalted 1 and 2E, Old Vampire Mage and Werewolf, New Vampire Werewolf and Changeling, as little 6d6 as I could get away with and lastly, FATE.

Oh, and some Traveller20 too.

Wait, how could I forget Call of Cthulu d20 myself? And the game of WFRP 3E I was playing?

It was an nWoD game. A bunch of mages "allied" with the vampires of Las Vegas and offered them a free feeding ground where their true nature couldn't be discovered and blood would be freely offered to anyone who wants it.

The only downside - the spell they used made every vampire look like they came from Twilight and every mortal woman into a Bella replica.

The specific example I am working with is the Sincerity (Honesty) skill in L5R but there is a certain factor of curiosity for any other system:
How do you really roll to tell the truth and logically fail?
After all, if someone is trying to "sense motive" (to use D&D 3.x terminology), and th...

So, it's a D&D context then. There isn't a skill for telling the truth. There is a skill for getting someone to do something because they are your friend (Diplomacy) or you'll hurt them (Intimidate) and a skill for persuading someone that something is true (Bluff).